Add interpolate_from_fr fun

[vihu]

This is just a helper function I ended up writing while working on an erlang nif for threshold_crypto. Thought it may be helpful to merge upstream.

[afck]

Not sure about this one: I don't think it's idiomatic in Rust to provide a Vec version for functions that take a slice.

If we add a generalization for convenience, I'd make it as general as possible, and actually modify compute_interpolation itself. It just iterates over the samples once anyway, so we could accept any T: IntoIterator where T::Item: Borrow<Fr>. That should make it accept vectors, slices and other iterators and collections.